The invention relates to a method of and an apparatus for the manufacture of dimensionally stable, liquid-impermeable, flexible, elastic press bands, particularly for use with broad-nip wet presses of paper making machines, said press bands comprising on their outer side, which faces the fibrous web or the felt, cavities and/or depressions suitable for liquid drainage, wherein a substantially bubble-free, free-flowing mixture of a prepolymer is cast onto a reinforcing band engaging the inner wall of a rotating centrifugal tube and is thereby positively joined to said reinforcing band. Press bends which are made by the specified method and the associated apparatus are known from DE-OS 3,224,760. Usually, they are coated by casting with a synthetic material, wherein the reinforcing band is partially embedded in the synthetic material.
According to such a method (DE-OS 3,318,984) the free-flowing mixture of a prepolymer is cast onto the reinforcing band, which is fitted onto a horizontal cylinder rotating about its longitudinal axis, in a casting jet which is slowly displaced in parallel to the longitudinal axis of the cylnder, or into a vertical or inclined casting mould in which the reinforcing band is provided in the form of a single- or multi-layer fabric composed of threads or wires of one or several materials such as polyamide, polyester, Kevlar, metal or the like or mixtures thereof.
This manufacturing process has the drawback that with such a coated press band the outer surface of the band is smooth whereas the inner surface thereof is provided with the cavities and/or depressions desirable for liquid drainage. For permitting a press band manufactured in this way to be used in the wet press, the band must first be reversed after removal from the casting mould so that the structured surface becomes the outer surface of the band while the smooth surface forms the inner surface thereof, because with such press bands the structured outer surface, which faces the fibrous web or the felt, is used for liquid drainage.
Especially with small band diameters, however, the reversing of such fully cast press bands is highly problematic and in any case requires much effort, and in addition to that the risk of damage to the bands cannot be excluded.
It is also known to grind drainage ducts into the outer surface of the press band, but this way of manufacturing is no less expensive.